Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars.<ref>Template:Cite news</ref> Bars are found in about two thirds of all spiral galaxies in the local universe,<ref name="eskridge"/> and generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well.<ref name="mihalas1968">Template:Cite book</ref><ref name="timothy1998">Template:Cite book</ref> The Milky Way Galaxy, where the Solar System is located, is classified as a barred spiral galaxy.<ref>Template:Cite journal</ref>
Edwin Hubble classified spiral galaxies of this type as "SB" (spiral, barred) in his Hubble sequence and arranged them into sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between the two. SB0 is a barred lenticular galaxy. A new type, SBm, was subsequently created to describe somewhat irregular barred spirals, such as the Magellanic Clouds, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are the spiral galaxy, elliptical galaxy and irregular galaxy.
Although theoretical models of galaxy formation and evolution had not previously expected galaxies becoming stable enough to host bars very early in the universe's history, evidence has recently emerged of the existence of numerous spiral galaxies in the early universe.<ref>Milky Way-like galaxy found in the early universe</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Bars
Barred galaxies are apparently predominant, with surveys showing that up to two-thirds of all spiral galaxies develop a bar.<ref name="eskridge">Template:Cite journal</ref> The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting farther out, which creates a self-perpetuating bar structure.<ref>Template:Cite journal</ref>
The bar structure is believed to act as a type of stellar nursery, channeling gas inwards from the spiral arms through orbital resonance, fueling star birth in the vicinity of its center.<ref>Template:Cite journal</ref> This process is also thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in the Southern Pinwheel Galaxy.
Bars are thought to be temporary phenomena in the lives of spiral galaxies; the bar structures decay over time, transforming galaxies from barred spirals to more "regular" spiral patterns. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Simulations show that many bars likely experience a "buckling" event in which a disturbance in the orbital resonances of stars in the bar structure leads to an inward collapse in which the bar becomes thicker and shorter, though the exact mechanism behind this buckling instability remains hotly debated.<ref>Template:Cite journal</ref> Barred spiral galaxies with high mass accumulated in their center thus tend to have short, stubby bars.<ref>Barred Spirals Come and Go, Sky and Telescope, April 2002</ref> Such buckling phenomena are significantly suppressed and delayed by the presence of a supermassive black hole in the galactic center but occur nonetheless.<ref>Template:Cite journal</ref>
Since so many spiral galaxies have bar structures, it is likely that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on average about two billion years.<ref>Ripples in a Galactic Pond Template:Webarchive, Scientific American, October 2005</ref>
Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. A 2008 investigation found that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with about 65 percent of their local counterparts.<ref name="ShethElmegreen2008">Template:Cite journal</ref>
Grades
The general classification is "SB" (spiral barred). The sub-categories are based on how open or tight the arms of the spiral are. SBa types feature tightly bound arms. SBc types are at the other extreme and have loosely bound arms. SBb galaxies lie in between. SBm describes somewhat irregular barred spirals. SB0 is a barred lenticular galaxy.
Examples
| Example | Type | Image | Information |
|---|---|---|---|
| NGC 2787 | SB0 | File:NGC 2787.jpg | SB0 is a type of lenticular galaxy |
| NGC 4314 | SBa | File:NGC 4314HST1998-21-b-full.jpg | |
| NGC 4921 | SBab | File:NGC 4921 by HST.jpg | |
| Messier 95 | SBb | File:Messier95 spitzer.jpg | |
| NGC 3953 | SBbc | File:NGC3953HunterWIlson.jpg | |
| NGC 1073 | SBc | File:Barred spiral galaxy NGC 1073 (captured by the Hubble Space Telescope).tif | |
| Messier 108 | SBcd | File:Messier108.jpg | |
| NGC 2903 | SBd | File:NGC 2903 GALEX.jpg | |
| NGC 5398 | SBdm | File:NGC 5398SST.jpg | SBdm can also be considered a type |
| NGC 55 | SBm | File:Phot-14a-09-fullres.jpg | SBm is a type of Magellanic spiral (Sm) |
Other examples
See also
- Galaxy morphological classification
- Galaxy formation and evolution
- IC 1011
- IC 3447
- Ceers-2112 galaxy
- Lenticular galaxy
- Firehose instability
References
External links
- Britt, Robert Roy. "Milky Way’s Central Structure Seen with Fresh Clarity." SPACE.com 16 August 2005.
- An article about the Spitzer Space Telescope's Milky Way discovery
- Devitt, Terry. "Galactic survey reveals a new look for the Milky Way." 16 August 2005.
- The original press release regarding the article above, from the University of Wisconsin
- 'Barred' Spiral Galaxy Pic Highlights Stellar Birth." SPACE.com 2 March 2001.
- Hastings, George and Jane Hastings. Classifying Galaxies: Barred Spirals, 1995. "Astronomers Find Multiple Generations of Star Formation in Central Starburst Ring of a Barred Spiral Galaxy." January 15, 2000.
- A press release concerning NGC 1326
- Barred spirals come and go Sky & Telescope April 2002.
- "ESO Provides An Infrared Portrait of the Barred Spiral Galaxy Messier 83." November 29, 2001.
- A press release from the European Southern Observatory.
- Horton, Adam. "Spitzer NGC 1291 barred spiral galaxy seen in infrared." 22 October 2014.